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CATALYST REMOVAL IN l-ALKENE POLYMERIZATION PROCESS Filed Dec. 51, 1958Nov. 28, 1961 Melvem 6. Hoff BY William Resnic/r RIVEY a R W... mM 0 m E3%35 mm mm um I w gtxmwmfifi vw 3w Q .Gfifiwu 9v t gmam A m xmmuwm A awE 8 E5 l vGQl mm mm, W ERR? :SE h3 8 $3M Q Q QEQM Q km 2 QEQRQQ tw mammm R. w) wEiEw 331M348 Patented Nov. 28, E961 Pie Filed Dec. 31, 1958,Ser. No. 784,182 Claims. (Cl. 26088.2)

This invention relates to polymerization of normallygaseous l-alkenes ina liquid medium with heterogeneous catalysts to produce a solution of anormally-solid polymer and dissolved l-alkene, and more particularly, tothe improvement wherein a polymerization inhibitor is added to thesolution so that catalyst can be readily removed therefrom underpolymerization conditions.

The present invention is specific to polymerization processes employingsolution polymerization conditions. In such processes a normally-gaseousl-alkene, e.g., ethylene and/or propylene dissolved in a liquid medium,e.g., a hydrocarbon oil, is contacted in the presence of a heterogeneouscatalyst to produce a solution of normallysolid polymer of said l-alkeneand dissolved l-alkene. A particularly-diflicult process problemencountered at this point is the removal of catalyst particles from thepolymer solution. To achieve substantially-complete removal of catalystparticles at high removal rates by liquidsolid separation techniquessuch as filtration, the solution should have as low a viscosity aspossible. Low viscosity is favored by operating at elevated temperature,e.g., temperatures in the l-alkene polymerization range, and byretaining the l-alkene in the solution during catalyst removal.

Retention of the l-alkene in the solution at polymerization conditionsleads to a number of process difficulties. For example, it results inadditional polymerization of l-alkene, bringing about undesiredvariations in product quality and/ or catalyst separation difficulties,particularly plugging of filters. Specifically, if temperature in thecatalyst separation step differs from the particular reactiontemperature employed during the reaction step, the additional polymerformed differs in molecular weight from that desired, for example, lowertemperatures resulting in higher molecular weight polymers. Moreover, itwould be difiicult to control conditions during the separation step atreaction temperature because the polymerization reaction is exothermicand a filter or centrifuge, as contrasted to a reactor, does not lenditself to fine temperature control. Poor temperature control maypossibly lead to thermal degradation of polymer. Furthermore, polymerformed during catalyst separation tends to agglomerate with catalystparticles, thereby causing separation difiiculties.

It is therefore an object of the present invention to remove catalystparticles from a l-alkene polymer solution at high removal rates. It isanother object of the present invention to separate catalyst from acatalyst-containing solution of dissolved l-alkene, polymerizedl-alkene, and solvent at polymerization conditions without encounteringthe aforementioned difliculties. These and other objects of the presentinvention will become apparent as the detailed description thereofproceeds.

It has now been discovered that solid catalyst particles can be removalfrom solutions of normally-solid l-alkene polymers at high removal ratesby retaining the l-alkene in the solution, in contrast to prior flashingof the l-alkene, by adding a polymerization inhibitor to the solution,and by filtering and/or centrifuging the inhibited solution atpolymerization conditions. Once the catalyst particles have beenremoved, the polymer product can be readily recovered from the solutionby, for example, flashing substantially all of the l-alkene and at leasta substantial I proportion of the solvent from the solution, and byremoving any residual solvent and/or grease from the polymer bypolymer-recovery techniques of the art.

As previously indicated, it is an essential feature of the presentinvention that solution polymerization conditions be employed, i.e.,temperature and pressure during reaction are such that both unreactedl-alkene and polymerized l-alkene remain dissolved in the solvent. Theparticular temperature and pressure selected within solutionpolymerization conditions depend upon a number of factors including theparticular catalyst system employed and the optimum conditions for suchsystem within solution polymerization conditions, the particularl-alkene being polymerized, the particular solvent selected, and thelike. In general, however, a temperature of at least about 300 F. isemployed in all cases. For example, when polymerizing ethylene with asodium-promoted molybdena-on-alumina catalyst with purified n-decane assolvent, a temperature of about 450570 F. and a pressure of about8004200 p.s.i.g. may be employed. Similarly, when polymerizing ethylenewith unpromoted chromia-on-silica-alumina catalyst with purified methylcyclo-hexane as solvent, a temperature of 300-350 F. and a pressure of400-600 p.s.i.g. may be employed.

While, as above indicated, the present invention is specific to solutionpolymerization conditions, it is otherwise applicable to various solidcatalyst systems effective for polymerizing l-alkenes, particularlyethylene, propylene and mixtures thereof. Specifically, the improvementis applicable to heterogeneous catalysts comprising essential-ly acompound of a transition metal selected from the subgroups 4, 5, and 6of the periodic system. The catalysts may be preformed solid catalysts,promoted preformed solid catalysts, precipitated solid catalysts, orpretreated precipitated solid catalysts. (Solid Polymers From SurfaceCatalysts, by Friedlander and Resnick, Advances in Petroleum Chemistryand Refining, volume 1, pages 526570, copyright 1958 by IntersciencePublishers, Inc., New York.)

In the preformed solid catalyst system, various metal oxides on highsurface supports are employed, preferably molybdenum oxide,cobalt-molybdate, or other metal molybda-tes supported on high surfacegamma-alumina. The catalyst is activated by partial reduction withreducing gases, e.g., hydrogen, carbon monoxide, or the like. Oxides ofother metals, e.g., magnesium, nickel, zinc, chromium, vanadium, orthorium can be present in pro portions up to about 10 Weight percent ofthe total catalyst. Other highsurface supports, e.g., titania, zirconia,or the like, can also be used. Other preformed catalysts includechromium oxide on various supports, e.g., silica, alumina, zi-rconia,thoria, or mixture thereof, preferably silica-alumina. Thechromium-oxide catalysts are activated by treating with dry air atatmospheric pressure. Chromium-oxide-on-silica-alumina can be promotedwith small amounts of strontium oxide.

The preformed solid catalysts can be greatly promoted by active metalsand hydrides. For example, catalytic activity of group VI metal oxidesis promoted by alkali metals, alkali metal hydrides, alkaline earthmetals, alkaline earth metal hydrides, metal borohydridcs, metalaluminohydrides, or calcium, strontium or barium carbides. The promotingaction of alkali metals may be increased by a small quantity of hydrogenhalide or alkyd halide. The activity of group V metal oxides, e.g.,vanadia, niobia, or the like, is promoted by alkaline earth hydrides,metal aluminohydrides, metal borohydrides, or metal alkyls. The activityof nickel or cobalt on charcoal is promoted by alkali metals or alkalineearth hydrides, such as calcium, barium, or magnesium hydrides,

The most important of the precipitated solid catalysts for. ethylenepolymerization are formed by the interaction of aluminum alltyls oraluminum sesquihalides with transition metal salts, such as titaniumtetrachloride. The pretreated precipitated solid catalyst systemsinclude the combination of a transition metal halide prereduced to alower valence state and an active organemetallic compound, such asaluminum trialkyl. Titanium trichloride, titanium dichloride, andvanadium trichloride are commonly used.

The solvent employed may be any substantially inert liquid, but it ispreferably a hydrocarbon. Of the various hydrocarbon solvents, it' ispreferred to employ a parafi'inic solvent containing an average of about9 or 10 carbon atoms per molecule. Aromatic solvents such as benzene,toluene, or xylene may be used, but they tend to interfere with removalof trace color bodies or color-forming materials which may be producedduring polymerization. Furthermore, aromatic solvents tend to becomealkylated and converted into undesirable byproduct materials such asgrease. Paraflins which are much-higher boiling than C are: morediflicult to remove from polymer product. On the other hand, lowboilingparaffin solvents may result in the formation of two liquid phases andmay require increased equipment costs. The parafiinic solvent mayinclude cyclo-paraffins (naphthenes), and for practical purposes anarrow kerosene fraction boiling in the range of about 360 to 400 F. isusually suitable.

The particular polymerization inhibitor, which is added to thepolymerization zone effiuent, and the amount thereof depend in part onthe particular catalyst system employed. Some inhibitors, however, canbe used in a wide variety of the above-described catalyst systems. Forexample, water, carbon dioxide and/or oxygen inhibits the polymerizationactivity of preformed catalysts, e.g., chromia-on-silica-alumina, thepromoted preformed catalysts, e.g., sodium-promotedmolybdena-on-alumina, or the precipitated catalysts, e.g., aluminumtrialkyl and titanium tetrachloride. Addition of water as an inhibitormay, however, lead to color difiiculties in the polymer product. Variousother inhibitors can alternatively be used. For example, in the case ofsodium-promoted molybdena-on-alumina catalyst (and also precipitatedcatalysts) effective inhibitors have been found to include acetic acid,phenol, benzylamine, diethylamine, triethylamine, diphenylamine (but nottrip-henylamine), etc. Typically, about 0.01 to 5 moles of inhibitor isadded per mole of sodium, a typical catalyst containing 0.02 mole ofsodium per gram of molybdena-on-alumina. The particular inhibitor andthe amount thereof is not, per se, part of the present invention.

The invention will become more clearly understood from the followingdetailed description of a particular embodiment read in conjunction withthe accompanying figure, which forms a part of this specification andwhich is a simplified schematic flow diagram of a poly ethylene processutilizing the present invention.

Referring to the figure, ethylene, solvent, and catalyst from sources 7,8, and 9 respectively are charged to ethylene purifier 10, solventpurifier 11, and catalyst preparation zone 12 respectively. Ethylenepurification may be efiected, for example, by passing the ethylenethrough a caustic scrubber to remove carbon dioxide, a Water scrubber toremove caustic, and a gas dryer to remove any traces of water. Any knowntype of purification system may be employed, but it is important thatthe ethylene be substantially free from oxygen (less than 50 andpreferably less than 10 p.p.m.). Its moisture content should be at leastlow enough to correspond to a -50 F. dew point and it should besubstantially free from C0 and CO While ethylene is the l-alkene whichis polymerized in this example, it should be understood that theinvention is applicable to the polymerization of other l-alkenes, e.g.,propylene, normal butenes, and the like and/or mixtures thereof. It is,however,

most advantageously applied to ethylene, propylene, and/ or mixturesthereof.

Solvent, which in this example is a narrow kerosene fraction boiling inthe range of 360 to 400 F. and is normally recycled for economy reasons,is purified in solvent purifier 11 by, for example, heart-cutdistillation to remove heavy and light ends, by azeotropic distillationto remove water, and by passing the solvent through a bed packed withadsorbent, such as, charcoal, activated alumina, silica gel, syntheticzeolite of 4 angstrom pore openings, and/or the like, preferably silicagel. in this example, the catalyst is sodium-promotedmolybdena-on-alumina which is slurried in catalyst preparation zone 12with purified solvent introduced from solvent purifier 11 via lines 13and 14. The sodium and molybdena-on-alumina may be dispersed separatelyinto the purified solvent. Alternatively, the sodium may be dispersed onthe surface of :molybdena-on-alumina in the form of high-surface sodium,which in turn may be dispersed in purified solvent.

The uniform slurry of catalyst and solvent is charged via line 15 toreactor 16. Similarly, purified ethylene is introduced from ethylenepurifier 10 to reactor 16 via line 17 and purified solvent is introducedvia lines 13 and 18. Alternatively (but not shown in the figure), thepurified ethylene may be introduced directly into line 18 from line 17and the resulting solution of ethylene in solvent introduced intoreactor 16. Polymerization in reactor 16 may typically be effected at atemperature of about 480 F. and a pressure of about 1,000 p.s.i.g.,corresponding to solution polymerization condi tions.

Reactor 16 is provided with an impeller-type mixer to assure adequatecontacting of dissolved ethylene with the finely-divided catalyst. In abatch-type reactor system a fine catalyst, e.g., up to 100 mesh (ASTMDesignation E1 l-39, 1949), may be employed, preferably in the form ofmicrospheres. In a continuous reactor system (and optionally in abatch-type reactor system) relatively large-size catalyst particles inthe range of about 20-80 mesh maximizes retention of catalyst particlesin the reactor, and subsequent polymer purification is thereby somewhatsimplified. In such reactors, a screened baille may be interposed at thepolymer solution outlet in order to further maximize retention of thecoarse catalyst in the reactor. A catalyst concentration in the reactorof the order of 10 weight percent or more, or approximately 0.7 poundper gallon of stirred reactor volume, is desirable although catalystconcentrations may be much lower or higher. Ethylene and solvent enterthe reactor from lines 17 and 18 at about 400 F.; and as, above stated,the reactor is operated at about 450 F. and about 1,000 p.s.i.g.

Reactor eifiuent, which contains solvent, unreacted ethylene dissolvedtherein, dissolved polyethylene and catalyst particles, is withdrawnthrough line 19 and passes via lines 20 and 21 to catalyst separatorzone 22. Alternatively, efiluent from line 19 may be passed to catalystseparator 22 via line 20a and heat exchanger 201; if temperature of theeffluent is to be adjusted prior to catalyst separation. Prior toentering catalyst separator 22 effiuent from reactor 16, is inhibited inlines 20 or 20a by addition of an ethylene polymerization inhibitor fromline 23. While, as previously stated, various polymerization inhibitorsmay be used, in this example the inhibitor is moist carbon dioxide whichis introduced at the rate of about 0.1 mole per mole of sodium promoterin theeffluent. The moist carbon dioxide effectively nullifies thesodium promoter so that the molybdena-on-alumina catalyst is essentiallydead for polymerization.

. Catalyst separator 22 may be a conventional liquidsolid separator,such as a filter or centrifuge or combination thereof, the effectivenessof which for catalyst separation is greatly enhanced by the very-lowviscosity of the material entering catalyst separator 22 via hue 21.

While very-low viscosity increases the effectiveness of centrifuges, thepresent invention is most advantageously employed in solving separationproblems associated with filters, e.g., low-filter rates, filterplugging, and the like. The low viscosity is achieved by operatingcatalyst separator 22 under pressure conditions assuring retention ofethylene in solution and at elevated temperature. Conveniently theseconditions are essentially the same as in reactor 16, in which case apump (not shown) may be interposed between the reactor and catalystseparator. Catalyst separator 22 may also be operated at a slightlylower pressure than reactor 16 so as to facilitate the flow of eflluentfrom reactor 16 to catalyst separator 22 without additional pumping. Insuch case, temperature in catalyst separator 22 may be lowered somewhat,if necessary, to prevent flashing of ethylene during catalystseparation.

Catalyst separator 22 may be a conventional centrifugal-type separator,e.g., a Merco centrifuge (registered trademark, Dorr-OliverIncorporated, Stamford, Connecticut) or a conventional plate-and-framefilter (optionally with heated frames) employing paper (preferablyhighwet-strength cellulosic type) or cloth filter media. Alternatively,catalyst separator 22 may be a Sparkler Filter (vertical or horizontalpressure leaf filter, Sparkler Manufacturing Company, Mundelein,Illinois) or a filter-aid precoat filter. If ordinary paper filters weemployed in catalyst separator 22, temperature of thecatalyst-containing solution entering catalyst separator 22 may have tobe lowered sufficiently in heat exchanger 21b to at least the maximumtolerable temperature, e.g., about 400 F., that the paper can standbefore effective destruction thereof as a filter medium.

Because of the addition of polymerization inhibitor from line 23, nofurther polymerization occurs in catalyst separator 22 even though it isoperated at polymerization conditions. Thus, a very low viscosity isachieved with resultant very high separation rates without plugging orother separation difficulties caused by rafter-polymerization. Moreover,the temperature in catalyst separator 22 can be varied from reactortemperature, i.e., lowered or raised in heat exchanger 21b, withoutproducing a difierent molecular weight polymer than that alreadyproduced in reactor 16. Thus, temperatures can be raised in catalystseparator 22 to increase separation rates still further (limited, ofcourse, by polymer thermal-degradation considerations). A temperaturerise of as little as 50-60 F. may as much as double filter rates.Temperature may also be lowered somewhat to permit use of atemperature-sensitive filter medium.

After removal of catalyst, the solution of ethylene, polymer, andsolvent is charged via line 24, heat exchanger 25 and line 26 to flashzone 27, wherein substantially all of the ethylene and at least asubstantial portion of the solvent are flashed overhead via line 28 andheat exchanger 29 to separator 30. Uncondensed gases and vapors arepassed from separator 30 via line 31 and heat exchanger 32 to separator33. Condensate from separator 30, i.e., solvent, is removed via line 34and condensate from line 33, also solvent, is removed via line 35. Thiscondensate is combined in line 36 and is ultimately recycled, along withmakeup solvent, via line 8 to solvent purifier 11. Purification ofrecycled solvent is usually essential because of gradual degradationthereof during use. Overhead from separator 33, which issubstantially-pure ethylene, is removed via line 37 and is also recycledbut need not necessarily be repurified and thus can be introduceddirectly to line 17.

Bottoms from flash zone 27, which may be substantially purepolyethylene, but which also may optionally contain 5 to 25% solvent tofacilitate pumping, are removed via line 38 and transferred to a polymerrecovery and/or purification zone 39. Various techniques are availablefor recovery and/or purification, the particular finishing operationsper so not being a part of the present invention. They may include steamdistillation, gas stripping, vacuum extrusion, solvent (e.g., hexane)extraction, expression, pellctization and/ or the like.

From the foregoing description it is evident that addition ofpolymenization inhibitor in accordance with the present inventionresults in improved operation and great simplification of the process.First and foremost, high catalyst-recovery rates are achieved in thecatalyst separator without encountering plugging or other separationdiiTiculties and without additional polymerization which may causeoff-specification product and/or uncontrolled temperatures in thecatalyst separator (from the exothermic nature of the reaction). Inaddition, the present inventionpermits great process simplificationwhich results in substantial operating and investment savings. Forexample, only one operation, e.g., a single flashing operation, isrequired for simultaneous removal of both ethylene and solvent, whereasmethods of the prior art usually employ at least two steps.

While the invention has been described by reference to a particularexample thereof, it should be understood that the invention is alsoapplicable to the use of other catalysts such as, for example,chromia-onsilica-alumina or titanium chloride-aluminum alkvl systems.Patentable novelty is not claimed .in the catalyst compositions orreaction conditions per so since these are well known to those skilledin the art.

Having thus described the invention, what is claimed is:

1. A process which comprises (1) polymerizing a normally gaseousl-alkene monomer in the presence of a preformed solid metal oxidecatalyst selected from oxides of transition metals from the groupconsisting of groups V and VI of the periodic system at elevatedtemperature and pressure effective to produce a normally-solid polymeras a solute dissolved in a liquid reaction mixture comprising asubstantially inert liquid solvent and a substantial amount of excessunreacted l-alkene monomer, (2) introducing into said reaction mixture asmall amount, effective to inhibit further polymerization, of carbondioxide, and (3) separating the preformed solid metal oxide catalyst asa solid from said liquid reaction mixture, said separation beingcharacterized by effecting it at elevated pressure to retain said excessl-alkene monomer in the liquid phase, and by effecting it at elevatedtemperature, to maintain a low viscosity of said liquid reactionmixture.

2. Process of claim 1 wherein said l-alkene is ethylene.

3. Process of claim 1 wherein said l-alkene is propylene.

4. Process of claim 1 wherein said l-alkene is a mixture of ethylene andpropylene.

5. Process of claim 1 wherein said metal oxide is chromia.

6. Process of claim 1 wherein said metal oxide is molybdeua.

7. Process of claim 1 wherein a catalyst promoter is present in thereaction mixture.

8. Process of claim 1 wherein said carbon dioxide is moist carbondioxide.

9. Process of claim 1 wherein said separation is effected with a filter.

10. Process of claim 1 wherein said separation is effected with acentrifuge.

References Cited in the file of this patent UNITED STATES PATENTS2,825,721 Hogan et a1 Mar. 4, 1958 2,827,447 Nowlin et a1 Mar. 18, 19582,845,412 Heyson July 29', 1958 2,886,561 Reynolds et al May 12, 19592,890,214 Brightbill et a1 June 9, 1959

1. A PROCESS WHICH COMPRISES (1) POLYMERIZING A NORMALLY GASEOUS1-ALKENE MONOMER IN THE PRESENCE OF A PREFORMED SOLID METAL OXIDECATALYST SELECTED FROM OXIDES OF TRANSITION METALS FROM THE GROUPCONSISTING OF GROUPS V AND VI OF THE PERIODIC SYSTEM AT ELEVATEDTEMPERATURE AND PRESSURE EFFECTIVE TO PRODUCE A NORMALLY-SOLID POLYMERAS A SOLUTE DISSOLVED IN A LIQUID REACTION MIXTURE COMPRISINGING ASUBSTANTIALLY INERT LIQUID SOLVENT AND A SUBSTANTIAL AMOUNT OF EXCESSUNREACTED 1-ALKENE MONOMER, (2) INTRODUCING INTO SAID REACTION MIXTURE ASMALL AMOUNT EFFECTIVE TO INHIBIT FURTHER POLYMERIZATION, OF CARBONDIOXIDE, AND (3) SEPARATING THE PERFORMED SOLID MET AL OXIDE CATALYST ASA SOLID FROM SAID LIQUID REACTION MIXTURE, SAID SEPARATION BEINGCHARACTERIZED BY EFFECTING IT AT ELEVATED PRESSURE TO RETAIN SAID EXCESS1-ALKENE MONOMER IN THE LIQUID PHASE, AND BY EFFECTING IT AT ELEVATEDTEMPERATURE, TO MAINTAIN A LOW VISCOSITY OF SAID LIQUID REACTIONMIXTURE.